This invention relates to maize improvement. More specifically, this invention relates to an inbred maize line designated PSA104.
Maize
Maize or corn (Zeays mays L.) is a major annual crop species grown for grain and forage. A monocot, maize is a member of the grass family (Gramineae) and bears seeds in female inflorescences (usually called ears) and pollen in separate male inflorescences (usually called tassels).
In the U.S., maize is almost exclusively produced by growing hybrid varieties (cultivars). Maize hybrids are typically produced by seed companies and sold to farmners. On farms, maize hybrids are usually grown as a row crop. During the growing season herbicides are widely used to control weeds; fertilizers are used to maximize yields; and fungicides and insecticides are often used to control disease pathogens and insect pests. Before maturity, maize plants may be chopped and placed in storage where the chopped forage (stover) undergoes fermentation to become silage for livestock feed. At maturity in the fall, the seeds are harvested as grain. The grain may be directly fed to livestock or transported to storage facilities. From storage facilities, the grain is transported to be used in making an extremely large number of products, including food ingredients, snacks, pharmaceuticals, sweeteners, and paper products (see, e.g., S. A. Watson and P. E. Ramstad, Eds., Corn: Chemistry and Technology, American Association of Cereal Chemists, Inc., St. Paul, Minn. (1987)).
While the agronomic performance of maize hybrids has improved, there is a continuing need to develop better hybrids with increased and more dependable grain and stover yields. Moreover, heat and drought stress and continually changing insect predators and disease pathogens present hazards to farmers as they grow maize hybrids. Thus, there is a continual need for maize hybrids which offer higher grain yields in the presence of heat, drought, pathogens and insects.
Inbred Lines and Hybrid Varieties
The ultimate purpose for developing maize inbred lines is to be able to dependably produce hybrids. Commercially viable maize hybrids, like hybrids in many other crop species, manifest heterosis or hybrid vigor for most economically important traits.
Plants resulting from self-pollination (or from other forms of inbreeding) for several generations are termed inbreds (inbred lines). These inbreds are homozygous at almost all loci. When self-pollinated, these inbreds produce a genetically uniform population of true breeding inbred progeny. These inbred progeny possess genotypes and phenotypes essentially identical to that of their inbred parent. A cross between two different inbreds produces a genetically uniform population of hybrid F1 plants. These F1 plants are genetically uniform, but are highly heterozygous. Progeny from a cross between two hybrid F1 plants are also highly heterozygous, but are not genetically uniform.
One important result of this phenomenon is that seed supplies of an inbred may be increased by self-pollinating the inbred plants. Equivalently, seed supplies of the inbred may be increased by growing inbred plants such that only pollen from these inbred plants is present during flowering (anthesis), e.g., in spaced or timed isolation. Seed arising from inbred parents successfully grown in isolation is genetically identical to the inbred parents. Another important result is that hybrids of inbred lines always have the same appearance and uniformity and can be produced by crossing the same set of inbreds whenever desired. This is because inbreds, themselves, are genetically uniform. Thus, a hybrid created by crossing a defined set of inbreds will always be the same. Moreover, once the inbreds giving rise to a superior hybrid are identified, a continual supply of the hybrid seed can be produced by crossing these identified inbred parents.
Types of hybrids include single-cross, three-way, and double-cross. Single-cross hybrids are the F1 progeny of a cross between two inbred lines (inbreds), e.g., Axc3x97B, in which A and B are inbreds. Three-way hybrids are the first generation progeny of a cross between a single-cross hybrid and an inbred, e.g., (Axc3x97B)xc3x97C, in which Axc3x97B is a single-cross hybrid of inbreds A and B and C is another inbred. Double-cross hybrids are the first generation progeny of a cross between two single-cross hybrids, e.g., (Axc3x97B)xc3x97(Cxc3x97D), in which Axc3x97B and Cxc3x97D are single-cross hybrids of inbreds A and B and C and D, respectively. In the U.S., single-cross hybrids currently occupy the largest proportion of the acreage used in maize production. As will be shown below, maize inbreds are assemblages of true breeding, homozygous, substantially identical (homogeneous) individuals. Single-cross hybrids are both homogeneous and highly heterozygous and are not true breeding. Three-way and double-cross hybrids are less homogeneous, but are nonetheless highly heterozygous and not true breeding as well. Hence, the only way of improving hybrids is improving component inbreds thereof. Improving maize inbreds involves procedures and concepts developed from the discipline of plant breeding.
Plant Breeding
Developing improved maize hybrids requires the development of improved maize inbreds. Maize breeding programs typically combine the genetic backgrounds from two or more inbred lines or various other broad based germplasm sources into breeding populations from which new inbred lines are developed by self-pollination (or other forms of inbreeding) and selection for desired phenotypes. The newly developed inbreds are crossed to other inbred tester lines and the hybrids from these tester crosses are then evaluated to determine whether these hybrids might have commercial potential. Thus, the invention of a new maize variety requires a number of steps. As a nonlimiting illustration, these steps may include:
(1) selecting plants for initial crosses;
(2) crossing the selected plants in a mating scheme to generate F1 progeny;
(3) self-pollinating the F1 progeny to generate segregating F2 progeny;
(4) sequentially self-pollinating and selecting progeny from the F2 plants for several generations to produce a series of newly developed inbreds, which breed true and are highly uniform, yet which differ from each other;
(5) crossing the newly developed inbred lines with other unrelated inbred lines (testers) to produce hybrid seed; and
(6) evaluating the tester hybrids in replicated and unreplicated performance trials to determine their commercial value.
Plants are selected from germplasm pools to improve hybrid traits such as grain and stover yield, resistance or tolerance to diseases, insects, heat and drought, stalk quality, ear retention, and end use qualities. The plants from the germplasm pools are then crossed to produce F1 plants and the F1 plants are self-pollinated to generate populations of F2 plants. Self-pollination and selection in F2 plants and subsequent generations are illustrated below in a nonlimiting example of a pedigree method of breeding.
In the nursery, F2 plants are self-pollinated and selected for stalk quality, reaction to diseases and insects, and other traits, which are visually scored. During the next growing season, seeds from each selected self-pollinated F2 plant are planted in a row and grown as F2-derived, F3 families. Selection and self-pollination is practiced among and within these F3 families. In a subsequent growing season, seeds from each selected F3 plant are planted in a row and grown as F3-derived, F4 families. Selection and self-pollination are again practiced among and within these F4 families. In a subsequent growing season, seeds from each selected F4 plant are planted in a row and grown as F4-derived, F5 families. At this point, selection is practiced predominantly among families, rather than within families, because plants within families tend to be uniform and are approaching homozygosity and homogeneity. Seeds from selected F5 plants are harvested to be further selected for uniformity prior to being increased.
Simultaneous with self-pollination and selection, seeds from each selected F3, F4, and F5 plant are planted in a female row in one or more isolation blocks along with rows planted with seed of a tester (male) inbred. These isolation blocks are often grown at winter locations so the seed harvested therefrom can be grown in performance trials during the next growing season. Prior to anthesis, tassels from the selected F3, F4, and F5 female plants are removed before they shed pollen, so that the only pollen present in the isolation block is from the tester inbred. Seeds arising from the selected F3, F4, and F5 female plants are hybrid seeds having the selected F3, F4, and F5 plants as maternal (seed) parents and the tester inbred as the paternal (pollen) parent.
Hybrid seeds from the isolation blocks, check hybrids, and commercially significant hybrids of the same maturity are grown in replicated performance trials at a series of locations. Each check hybrid is the result of crossing the tester parent and an inbred parent of known maturity and proven agronomic value. During the growing season, the hybrids are visually scored for any of the above-described traits. At maturity, plots in these trials are usually scored for the percentage of plants with broken or tilted stalks and dropped ears. At harvest, grain yield, grain moisture, and grain test weight are determined. The resulting data from these performance trials are analyzed by calculating means and other statistics. These other statistics (e.g., coefficients of variation, repeatability) provide indications of the reliability (precision) of the means obtained from the performance trials. Means from these performance trials are then used to further cull plants in the nursery on the basis of unsatisfactory performance of their hybrids. Performance trials for earlier generations typically evaluate more hybrids and are planted at fewer locations than performance trials for later generations. At some point, seed supplies of elite inbred candidates from the nursery are increased and are used to produce larger amounts of experimental hybrids. These experimental hybrids are evaluated in replicated performance trials at maximum possible numbers of locations and may be grown alongside commercial hybrids from other seed companies in farmer fields in unreplicated trials as well. If the experimental hybrids perform well with respect to the commercial hybrids in these replicated and unreplicated trials, they are commercialized.
While the above-described pedigree method is widely used to develop maize inbreds, variations are widely used as well. Moreover, other breeding method protocols such as those for bulks, backcrossing, recurrent selection, and mass selection may be practiced in addition to, or in lieu of, the pedigree method described above. Theories and exemplary protocols for the pedigree method, bulk method, recurrent selection, and mass selection are known to the art, but are disclosed in, e.g., A. R. Hallauer and J. B. Miranda Fo, Quantitative Genetics in Maize Breeding, Iowa State University Press, Ames, Iowa (1981); G. Namkoong, Introduction to Quantitative Genetics in Forestry, U.S. Dept. Agric. Forest Service Tech. Bull. No. 1588 (1979); F. N. Briggs and P. F. Knowles, Introduction to Plant Breeding, Reinhold Publishing Company, New York (1967), R. W. Allard, Principles of Plant Breeding, Wiley and Sons, New York (1960), N. W. Simmonds, Principles of Crop Improvement, Longman Group, Ltd., London (1979); and J. M. Poehlman, Breeding Field Crops, 2d Ed., AVI Publishing Co., Inc. Westport, Conn. (1979).
As discussed above, hybrids of promising advanced breeding lines are thoroughly tested and compared to appropriate check hybrids in environments representative of the commercial target area(s), usually for 2-3 years. The best hybrids identified by these performance trials are candidates for commercial exploitation. Seed of each of the newly developed inbred parents of these hybrids is further purified and increased in steps leading to commercial production. These prerequisite activities to marketing newly developed hybrids usually take from eight to 12 years from the time the first breeding cross is made. Therefore, development of new cultivars is a time-consuming process requiring precise planning and efficient allocation and utilization of limiting resources.
Identification of genetically superior individuals is one of the most challenging issues confronting the plant breeder. For many economically important traits, the true genotypic expression of the trait is masked by effects of other (confounding) plant traits and environmental factors. One method of identifying a superior hybrid is to observe its performance relative to other experimental hybrids and to a series of widely grown standard cultivars. However, because a single observation is usually inconclusive, replicated observations over a series of environments are necessary to provide an estimate of the genetic worth of a hybrid.
Maize is an important and valuable field crop. Hence, a continuing goal of plant breeders is to develop high-yielding maize hybrids, which are otherwise agronomically desirable and which are produced by stable inbred lines. To accomplish this goal, the maize breeder must continually develop superior inbred parent lines. Developing superior inbred parent lines requires identification and selection of genetically unique, superior individuals from within segregating populations.
Each segregating population is the result of a combination of a multitude of genetic crossover events, independent assortment of specific combinations of alleles at many gene loci, and inheritance of large groups of genes together due to the effects of linkage. Thus, the probability of selecting any single individual with a specific superior genotype from a breeding cross is infinitesimally small due to the large number of segregating genes and the virtually unlimited recombinations of these genes. Nonetheless, the genetic variation present among the segregating progeny of a breeding cross enables the identification of rare and valuable new genotypes. These rare and valuable new genotypes are neither predictable nor incremental in value, but are rather the result of expressed genetic variation. Thus, even if the genotypes of the parents of the breeding cross can be completely characterized and a desired genotype known, only a few, if any, individuals with the desired genotype may be found within a large, segregating F2 population. Typically, however, neither the genotypes of the parents used in the breeding cross nor the desired progeny genotype to be selected are known to any extent.
In addition to the preceding problem, it is not known with any degree of certainty how the new genotype would interact with the environment. This uncertainty is measured statistically by genotype-by-environment interactions and is an important, yet unpredictable, factor in plant breeding. A breeder of ordinary skill in the art can neither predict nor characterize a priori a new desirable genotype, how the genotype will interact with various climatic factors, or the resulting phenotypes of the developing lines, except perhaps in a very broad and gross fashion. A breeder of ordinary skill in the art would also be unable to re-create the same line twice from the very same original parents, because the breeder is unable to direct how the parental genomes will combine in the progeny or how the resulting progeny will interact with environmental conditions when undergoing selection. This unpredictability results in the expenditure of large amounts of limited research resources to develop each superior new maize inbred line.
A reliable method of controlling male fertility (pollen viability) in plants provides means for efficient and economical subsequent hybrid production. This is also the case when plant breeders are developing maize hybrids in breeding programs. All breeding programs rely on some sort of system or method of pollen control and there are several methods of pollen control available to breeders. These pollen control methods include barriers such as bags for covering silks and collecting pollen from individual plants, manual or mechanical emasculation (detasseling), cytoplasmic male-sterility (CMS), genetic male-sterility, and gametocides.
Hybrid maize seed is usually produced commercially by using a male-sterility system, manual or mechanical detasseling, or a combination of both. In typical commercial hybrid seed production, alternate strips of two maize inbreds are planted in a field. The tassels are removed from the inbred designated to be the seed or female parent. Alternatively, the female is male-sterile and is not detasseled. If there is sufficient isolation from sources of foreign maize pollen, the ears of the female inbred will be fertilized only with pollen from the other (male) inbred. The resulting seed, harvested from the female parents in a successful hybrid production effort, is hybrid F1 seed, which will germinate and grow into hybrid F1 plants.
Manual or mechanical detasseling can be avoided by using inbreds with cytoplasmic male-sterility (CMS). CMS requires both a homozygous nuclear locus and the presence of a cytoplasmic factor for sterility. Otherwise, the plant will produce viable pollen. The CMS system requires A-lines (females), B-lines (maintainers), and R-lines (males). Male-sterile A-lines are homozygous for a nuclear allele for pollen sterility and possess the cytoplasmic factor for pollen sterility as well. B-lines produce viable pollen because they are homozygous for the sterile nuclear allele, but possess a fertile cytoplasmic factor. With the exception for the allele for pollen fertility, B-lines usually have a nuclear genome essentially identical to their complimentary A-line. R-lines are homozygous for a nuclear allele for fertility and possess a fertile cytoplasmic factor. Thus, R-lines produce viable pollen. Seed of male-sterile A-lines is increased by being pollinated by complimentary B-lines. The resulting seed grows into male-sterile A-line plants because the fertile cytoplasmic factor from the B-lines is not transmitted by B-line pollen. Hybrid seed is produced by pollinating A-line plants with pollen from R-line plants. The resulting hybrid seed is heterozygous at the nuclear locus and possesses the sterile cytoplasmic factor. Thus, the hybrid seed will grow into plants which produce viable pollen.
In addition to CMS, there are several methods conferring genetic male-sterility. One method involves multiple loci (including a marker gene in one case) which confer male-sterility, as disclosed in U.S. Pat. Nos. 4,654,465 and 4,727,219 to Brar et al. Another method disclosed by U.S. Pat. Nos. 3,861,709 and 3,710,511 to Patterson uses chromosomal reciprocal translocations, deficiencies, and duplications. In addition to these methods, U.S. Pat. No. 5,432,068 to Albertsen et al., describes a system of induced nuclear male-sterility which includes: identifying a gene critical to male fertility; xe2x80x9csilencingxe2x80x9d this critical gene; replacing the native promoter from the critical gene with an inducible promoter; and inserting the genetically engineered gene back into the plant. The resulting plant is male-sterile while the inducible promoter is not operative because the male fertility gene is not transcribed. Fertility is restored by inducing the promoter with a non-phytotoxic chemical which induces expression of the critical gene, thereby causing the gene conferring male fertility to be transcribed. U.S. Pat. Nos. 5,689,049 and 5,689,051 to Cigan et al. disclose a transgenic maize plant rendered male-sterile by being transformed with a genetic construct including regulatory elements and DNA sequences capable of acting in a fashion to inhibit pollen formation or function.
Yet another male-sterility system delivers a gene encoding a cytotoxic substance into the plant. The cytotoxic substance is associated with a male tissue-specific promoter or an antisense system. In each instance, a gene critical to fertility is identified and an antisense transcription to that gene is inserted in the plant (see e.g., Fabinjanski, et al., EPO 89/3010153.8 Publication No. 329,308 and PCT Application No. PCT/CA90/00037 published as WO 90/08828).
Another system potentially useful to confer male-sterility uses gametocides. Gametocides are topically applied chemicals affecting the growth and development of cells critical to male fertility. Application of gametocides affects fertility in the plants only for the growing season in which the gametocide is applied. See, e.z., U.S. Pat. No. 4,936,904 to Carlson (N-alkyl-2-aryl-4-oxonicotinates, N-alkyl-5-aryl-4-oxonicotinates, N-alkyl-6-aryl-4-oxonicotinates, N-alkyl-2,6-diaryl-4-oxonicotinates). Inbred genotypes differ in the extent to which they are rendered male-sterile by gametocides and in the growth stages at which the gametocides must be applied.
During hybrid seed production, incomplete detasseling or incomplete inactivation of pollen from the female parent will cause some of the female parent plants to be self-pollinated. These selfed female plants will produce seed of the female inbred, rather than the desired hybrid seed. The selfed seed of the female plants will then be harvested and packaged along with the hybrid seed. Alternatively, seed from the male inbred line may also be present among hybrid seed if the male plants are not eliminated after pollination. In either case, once the mixture of hybrid and xe2x80x9cselfedxe2x80x9d seed is planted it is possible to identify and select the female or male inbreds growing among hybrid plants. Typically these xe2x80x9cselfsxe2x80x9d are easily identified and selected because of their decreased vigor for vegetative and/or reproductive characteristics (e.g., shorter plant height, small ear size, ear and kernel shape, or cob color). Identification of these selfs can also be accomplished through molecular marker analyses. See, e.g., Smith et al., xe2x80x9cThe Identification of Female Selfs in Hybrid Maize: A Comparison Using Electrophoresis and Morphologyxe2x80x9d, Seed Science and Technology 14:1-8 (1995). Through these technologies, the homozygosity of the self-pollinated line can be verified by analyzing allelic composition at various loci along the genome. These methods allow for rapid identification of the invention disclosed herein. See also, Sarca et al., xe2x80x9cIdentification of Atypical Plants in Hybrid Maize Seed by Postcontrol and Electrophoresis,xe2x80x9d Probleme de Genetica Teoritica si Aplicata Vol. 20(1): 29-42. As is apparent to one skilled in the art, the foregoing are only some of the ways by which an inbred can be obtained and seed supplies of inbreds and hybrids increased.
There is provided a seed of maize inbred line designated PSA104, a regenerable cell arising from the seed, a tissue culture arising from the regenerable cell, and a maize plant arising from the tissue culture. There is also provided a plant arising from said seed and pollen, ovules, and regenerable cells arising from the plant.
There is further provided a process of producing a maize seed, the process including identifying an inbred maize plant arising from said seed and disposed within an assemblage of hybrid maize plants and pollinating the inbred maize plant such that the maize seed arises therefrom. Pollinating may include self-pollinating and cross-pollinating.
There is yet further provided a process of sequentially inbreeding a maize plant, the process including inbreeding a hybrid maize plant and progeny thereof, one of the parents of the hybrid maize plant arising from the seed of the present invention. The process may further include planting the seed such that maize plants arise from the seed; inbreeding the maize plants such that seed arises from the maize plants; and harvesting the seed arising from inbreeding the maize plants. Planting, inbreeding, and harvesting may be cyclically continued until a family obtained from a plant arising from at least one of said inbred seed is substantially homogeneous.
There is still further provided a process of developing a derived maize plant. The process may include providing a maize plant arising from the seed of the present invention and introgressing a trait into the maize plant. Introgressing may include backcrossing, a tissue culture protocol inducing heritable somaclonal variation, and a transformation protocol. The transformation protocol may include microprojectile-mediated transformation, Agrobacterium-mediated transformation, electroporation, needle-like body-facilitated transformation, and any combination thereof.
According to the invention, there is provided a novel inbred maize line, designated PSA104. This invention thus relates to the seeds of inbred maize line PSA104, to the plants of inbred maize line PSA104, and to methods for producing a maize plant. The maize plant and seed of this invention may be produced by being crossed with itself or another maize line. This invention also includes methods for producing a maize plant containing one or more transgenes in its genetic material and to the derived (transgenic) maize plants produced by that method.
This invention also provides methods for producing other inbred maize lines from inbred maize line PSA104 and to the inbred maize lines derived by the use of those methods.
This invention further provides essentially homogeneous populations (assemblages) of inbred maize seed, designated as PSA104. Essentially homogeneous populations of inbred seed include seed of PSA104 and may be generally purified so as to be substantially free from other seed so that seed of PSA104 is present in these homogeneous populations in a proportion greater than between about 90% and 100%, 95% and 100%, or 100% of the total seeds. This proportion may further be greater than about 90%, 95%, 97.5%, 98.5%, 99%, or 100% of the total seeds present and may be measured by visual observation (e.g., growouts) and/or other protocols known to the art.
This invention still further provides hybrid maize seeds and plants produced by crossing the inbred line PSA104 with another maize line.
All publications, patents, patent applications, and other documents cited herein are hereby incorporated by reference in their entirety. In the case of conflict, the present specification shall prevail.